1. Field of the Invention
The present invention relates to a single mode surface emitting laser, and more particularly to a single mode surface emitting laser which has a characteristic of single transverse mode radiation in the broad region using reflectivity distribution of a reflector layer with antiguide clad.
2. Description of the Related Art
The vertical-cavity surface-emitting laser of semiconductor laser is used as an optical source for optical communication and parallel optical communication since it has merits of high coupling efficiency with optical fiber and easiness of two dimensional array manufacturing. However, the radiation characteristic of it should be improved in order to apply the vertical-cavity surface-emitting laser as a proper optical source of system. Particularly, a stable transverse mode characteristic is one of the most needed characteristics for the stable coupling with optical fiber and free space optical interconnection.
The vertical-cavity surface-emitting laser emits light in the vertical direction of the substrate and shows a single longitudinal mode characteristic in the longitudinal direction and multiple mode in the traverse direction in accordance with its characteristic of structure. This is caused by the fact the difference of loss between modes in the transverse direction is not large and spatial hole burn phenomena. There are two methods to improve such characteristics, one is to get a single transverse mode characteristic by making current injection region smaller than the mode width of beam and by preventing the occurrence of spatial hole burn, and the other is to control the radiation of higher order transverse mode by closing the light passing width with optical aperture. Also, there is a method of controlling the radiation by increasing the propagation loss of higher order transverse mode using the antiguide clad. For example, if the transverse mode characteristic is improved largely using amorphous GaAs as antiguide clad, it is possible to obtain single transverse mode characteristics to the size of a specified device. That is, the transverse mode is stabilized by the method of increasing the round trip loss of higher order transverse mode using the antiguide structure. However, it has some limitation to obtain stable transverse mode characteristics of broad region and large device using the antiguide only since the round trip loss decreases steeply as the device size increases.
Therefore, it is possible to improve the single mode radiation characteristic by controlling the spatial distribution of reflection of top reflector as well as antiguide clad and then increasing the loss of higher degree transverse mode without changing the loss of fundamental transverse mode in large.
In view of such aspect, it is an object of the present invention to provide a surface emitting laser which shows a single transverse mode radiation mode characteristic in a broad region in order to use it as an optical source for free space optical interconnection and optical fiber coupling.
A single mode surface emitting laser according to the present invention comprises an n-type semiconductor substrate which has an n-type lower electrode and an antireflection film under it, a laser pillar which is a stacked structure of bottom DBR, an active layer and a top DBR sequentially on the semiconductor substrate, a control layer of compound semiconductor whose energy gap is higher than its radiation peak, and which has different thicknesses spatially since the layer becomes thick on the center of the laser pillar and thin on the edge of it, an antiguide clad layer which is formed in the edge part of the laser pillar including the control layer and has higher refractive index than those of the active layer or the top DBR forming the laser pillar, and a top electrode which is formed on the antiguide clad layer and the control layer, and a insulation film between the antiguide clad layer and the top electrode.
A method for manufacturing a single mode surface emitting laser according to the present invention comprise the steps of forming a bottom DBR, an active layer, a top DBR and a control layer on an n-type semiconductor substrate sequentially, forming an antireflection film and an n-type bottom electrode under the semiconductor substrate, and etching the selected regions of the control layer, the top DBR, the active layer and the bottom DBR sequentially in order to form a laser pillar; forming a photo resistive film on the entire structure of the laser pillar, etching a part of the control layer at the same time of removing the photo resistive film so that the control layer has different thicknesses spatially by etching the control layer a little bit on the center part and etching deeply on the edge; and depositing an antiguide layer and a insulation film on the entire structure including the laser pillar sequentially, etching the insulation film and the antiguide clad layer to expose the control layer, and forming top electrode on the entire structure.